The present invention relates generally to a crash detection system and more particularly to a crash detection system utilizing multiple sensors with different performance characteristics. Crash detection systems in vehicles determine the occurrence and severity of a crash and, based upon the severity of the crash, activate the vehicle airbag. The crash detection system preferably correctly makes the determination whether to activate the airbag as early in the crash as possible. The earlier the airbag activation signal is generated, the more time is available to inflate the airbag. If the crash is detected earlier in the crash event, the airbag can be inflated more slowly and with less force, thereby reducing potential injury to the passenger from the airbag inflation itself. Increasing the sensitivity of the sensor will generate an airbag activation signal earlier in the crash, but detecting the occurrence of a crash early must be balanced against the danger of an unnecessary airbag activation.
The most commonly utilized crash detection sensor is a ball-in-tube xe2x80x9cBITxe2x80x9d sensor mounted at the forward end of a vehicle. Generally, the BIT sensor comprises a magnetic ball retained at one end of a tube by a permanent magnet. In the event of an impact, the ball is moved toward the opposite end of the tube away from the magnet toward a pair of electrical contacts. If the impact is sufficient, the ball overcomes the force of the magnet and closes the contacts electrically, generating the airbag activation signal. The BIT sensor is susceptible to generating an unnecessary airbag activation signal during minor fender benders, for which activation of the airbag would be undesirable. For example, the BIT sensor is susceptible to potentially generating an airbag activation signal in the event that the vehicle hits a deer. It is undesirable to deploy the airbag in a vehicle-deer collision, since the entire vehicle will not decelerate sufficiently to require the airbag deployment for the protection of the passenger. Further, deployment of the airbag in a vehicle-deer collision may cause the driver to lose control of the vehicle. In order to reduce the potential for unnecessary airbag activations in minor fender benders or deer collisions, the threshold of deceleration required to activate the BIT sensor is larger than would otherwise be desirable. This in turn causes the BIT sensor to activate the airbag later in the crash.
An improved crash detection sensor currently utilized generally comprises an electronic sensing module comprising an accelerometer sending an accelerometer signal to a CPU. The electronic sensor module is mounted near the center of the vehicle and is less susceptible to generating airbag activation signals in minor collisions, such as vehicle-deer collisions. However, the electronic sensing module may be susceptible to generating an airbag activation signal on a rough road. For that reason, the threshold of the electronic sensing module is increased above that which would otherwise be desirable. Again, this causes the airbag activation signal to be generated later in the crash.
The increased thresholds generally utilized in the BIT sensors and electronic sensing modules generally results in more time being required to detect a crash. As result, the airbag must then be activated more quickly with greater force, thereby increasing potential injury to the passenger by the airbag.
The present invention provides a crash detection system utilizing two crash detection sensors having different performance characteristics. An airbag activation signal is generated based upon an evaluation of signals from both crash detection sensors. In this manner, the potential for erroneous airbag activations is reduced, as is the time to properly detect a crash and the severity of the crash.
Preferably, the first sensor comprises a ball-in-tube sensor, generally as is known. The ball-in-tube preferably has a threshold lower than that generally utilized. The second sensor generally comprises an electronic sensing module with an accelerometer and CPU. The signals from the accelerometer and ball-in-tube sensor are sent to the CPU for evaluation. Based upon the signals from the first and second sensors, the CPU determines whether and when to activate the airbag and whether to activate multiple stages of the airbag.
Preferably, the CPU evaluates the acceleration signal from the accelerometer generally in the same manner as is currently known. The CPU analyzes the magnitude and shape of the accelerometer signal to determine the beginning of the crash, the severity of the crash and the type of crash. Based upon this, the CPU generates an accelerometer fire signal in a manner generally known currently, although preferably with a lower threshold.
The CPU then evaluates the type of crash and times required for the first and second sensors to generate fire signals to determine the severity of the crash. The CPU determines whether to activate the airbag, when to activate the airbag and whether to activate multiple stages of the airbag based upon the type of crash and the sum of the times from the beginning of the crash to the fire signals. Further, the CPU will not activate the airbag until both sensors have determined that the airbag should be activated, i.e. both sensors have generated fire signals. Because the two sensors have complementary performance characteristics regarding situations when the airbag should not be deployed, the thresholds of each sensor can be reduced in order to provide earlier crash detection. Since fire signals from both sensors are required before activation of the airbag, the complementary performance characteristics of the crash detection sensors will prevent firing of the airbag in undesirable situations.